import java.util.ArrayList;
import java.util.List;

class Solution16 {
    public String[] telephone = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    public List<String> ret;
    public StringBuffer path;
    public List<String> letterCombinations(String digits) {
        ret = new ArrayList<>();
        path = new StringBuffer();
        if(digits.length() == 0) return ret;
        dfs(digits,0);
        return ret;
    }

    public void dfs(String digits,int pos) {
        if(pos == digits.length()) {
            ret.add(path.toString());
            return;
        }
        String cur = telephone[digits.charAt(pos) - '0'];
        for(int i = 0; i < cur.length(); i++) {
            path.append(cur.charAt(i));
            dfs(digits,pos+1);
            path.deleteCharAt(path.length()-1);
        }
    }
}